Free fatty acids (FFA) have important effects on many cell systems. Elevated plasma FFA concentrations stimulate endogenous glucose production, cause abnormal VLDL secretion, inhibit muscle glucose uptake and oxidation, and alter vascular responsiveness. The elevated plasma FFA concentrations found in obesity and specifically upper body obesity are thought to play a significant role in the pathophysiology of obesity-related health problems. We measure FFA appearance (Ra) and disappearance (Rd) from the circulation using isotope dilution techniques to understand the mechanisms by which plasma FFA concentrations are maintained or altered and to relate energy availability to energy needs. Cells, however, most likely cannot detect FFA flux per se, but instead respond to the extracellular FFA concentrations to which they are exposed. The results of studies supported by this grant have led us to the unexpected conclusion that differences in FFA uptake can have substantial effects on systemic FFA concentrations. We found that at the same FFA concentrations and rates of fatty acid oxidation women have ~40% greater rates of FFA uptake and release. This can only be accounted for by greater non-oxidative FFA disposal. We also found that pioglitazone treatment of upper body obese adults resulted in greater FFA clearance (lower concentrations without lower lipolysis) during hyperinsulinemia. The major sites/mechanisms for non-oxidative FFA disposal include direct reuptake into adipose tissue, recycling via VLDL-triglyceride or uptake into intramyocellular triglyceride. Each of these pathways participates in non-oxidative FFA disposal, but the relative contribution of each pathway is unknown. Our preliminary data suggests that the pathways/mechanisms of non-oxidative FFA disposal are different in men and women and between lower body obese and upper body obese women and obese men. We have developed or adapted methods that allow partitioning of FFA disposal directly and indirectly into adipose tissue, directly into muscle and to follow FFA through the VLDL pool. We will determine what tissues and mechanisms account for non-oxidative FFA disposal in lean and obese men and women under the conditions typically encounter in daily life: resting, postprandial and during physical activity. The Specific Aims of this proposal are to determine whether: 1. Non-oxidative FFA disposal directly into adipose tissue differs between men and women in the overnight postabsorptive condition, under fed conditions and during physical activity. 2. Non-oxidative FFA disposal directly into intramyocellular triglyceride differs between men and women in the overnight postabsorptive condition, under fed conditions and during physical activity. 3. Non-oxidative FFA disposal via VLDL-triglyceride differs between men and women in the overnight postabsorptive condition, under fed conditions and during physical activity. 4. Obesity and body fat distribution alter the pathways/mechanisms of non-oxidative FFA disposal under overnight postabsorptive and fed conditions as well as during physical activity. 5. FFA availability or percent body fat, rather than sex determine the relative disposition of non-oxidative FFA disposal. The results of these studies should provide new insights into the determinants of plasma FFA concentrations in humans, which should in turn allow focused and productive studies of the basic mechanisms. Understanding these mechanisms may lead to new treatment approaches for obesity. The proposed studies represent a logical extension of our efforts to define the role of FFA metabolism in the adverse health consequences of obesity.